For the causes of pain in the posterior fossa of the leg, the reasons are as follows: 1, popliteal nerve injury: often caused when the leg stretching activities, such as the elderly leg press, if the knee joint is over-extended, it will cause the posterior popliteal nerve to be pulled, which will lead to popliteal nerve strain, the main manifestation of which is the pain in the posterior part of the knee joint. 2, popliteal cyst: mainly due to the joint fluid from the posterior joint capsule weak outflow The lumps are formed when the joint fluid accumulates in the back of popliteal fossa. After regular walking and exertion, the cyst will gradually become bigger and thus accumulate at the back of the knee joint, and you will feel swelling, pain and swelling. If a popliteal cyst develops, arthroscopic surgery can be performed to remove the cyst, along with a compression bandage to prevent recurrence of the cyst.